Five Insects Evolve Resistance to Engineered Seeds: Study

By Jack Kaskey -
Jun 10, 2013

Five of 13 major crop pests have
evolved resistance to corn and cotton genetically engineered to
make their own insecticide, providing lessons for extending the
usefulness of such technologies, University of Arizona
researchers said in a study.

The increase in resistance, from one insect species in
2005, was expected because the crops are more widely planted,
pests have been exposed to the insecticides for more years and
monitoring efforts have improved, according to the study
published today in the journal Nature Biotechnology. Some
technologies have kept resistance at bay for more than than 15
years while others succumb in as few as two years, the study
said.

More than 1 billion acres (405 million hectares) worldwide
have been planted with crops engineered to produce insecticidal
proteins derived from Bacillus thuringiensis, or Bt, a soil
bacterium, reducing use of chemical insecticides, the study
said. Where data shows large numbers of resistant pests are
living in fields, and where the resistant trait is not
recessive, regulators need to take stronger actions, the study
said.

“When risk is indicated, either take more stringent
measures to delay resistance, such as requiring larger refuges,
or this pest will probably evolve resistance quickly to this Bt
crop,” Bruce Tabashnik, a University of Arizona entomologist
and the paper’s lead author, said in a statement accompanying
the study.

Regulators try to delay resistance by requiring farmers who
plant Bt crops to also plant a “refuge” on adjacent land using
non-Bt crops. The reasoning is that bugs not exposed to the
toxin will mate with any resistant insects, creating a new
generation that is once again susceptible to the insecticide.

Armyworms, Borers

The study analyzed data from 77 studies on 13 pest species
on five continents. Three of the five cases of resistance are in
the U.S., where about half of the world’s Bt crop acreage is
planted.

Rootworms are the most recent insect to develop resistance,
overcoming Monsanto Co. (MON)’s rootworm-killing corn in at least two
U.S. states, the U.S. Environmental Protection Agency said in
January, confirming earlier university studies.

Other instances of field-evolved resistance in Bt corn
include fall armyworms in Puerto Rico, a U.S. territory, and
maize-stem borers in South Africa, the researchers said. Bt-resistant insects in cotton include pink bollworms in India and
bollworms in the U.S.

“Perhaps the most compelling evidence that refuges work
comes from the pink bollworm, which evolved resistance rapidly
to Bt cotton in India, but not in the U.S.,” Tabashnik said in
the statement. “Same pest, same Bt protein, but very different
outcomes.”

Indian farmer compliance with planting refuges “was low,”
while institutional support in the U.S. helped farmers implement
“an effective refuge strategy” for pink bollworms, he said.